Organic electroluminescent devices

ABSTRACT

The present disclosure relates to the field of display technologies, and particularly discloses an organic electroluminescent device. The organic electroluminescent device has a first electrode, a second electrode, and an organic functional layer. The organic functional layer comprises a light-emitting layer. The light-emitting layer comprises at least a host material and a guest material. The host material comprises an exciplex composed of a donor molecule and a receptor molecule, wherein the donor molecule and/or the receptor molecule contains a large steric hindrance substituent group X for increasing an inter-molecular distance between the donor molecule and the receptor molecule, which enables to enhance Foster energy transfer to the guest material molecule, improve device efficiency, inhibit Triplet-Polaron Annihilate Annihilation (TPA), and prolong the device lifetime.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No.PCT/CN2018/088881 filed on May 29, 2018, which claims priority toChinese patent application No.

201711302958.6 filed on Dec. 8, 2017. Both applications are incorporatedherein by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to the field of display technologies.

BACKGROUND

Organic electroluminescent devices (also called Organic Light-EmittingDiodes, OLED) have a great application prospect in the field of displayand illumination and are increasingly concerned by people due to theadvantages of ultra-thin, light weight, low energy consumption, activeillumination, wide viewing angle, and fast response.

SUMMARY

Therefore, the technical problems to be solved by the present disclosureare to overcome the large singlet-triplet energy level gap (ΔE_(ST)) ofan exciplex TADF host material, the low RISC rate k_(RISC), the severeTriplet-Polaron Annihilate (TPA) in the light-emitting layer, and theproblem that performances such as the device efficiency and the servicelife need to be further improved.

For this purpose, the present disclosure provides an organicelectroluminescent device, comprising a first electrode, a secondelectrode, and an organic functional layer located between the firstelectrode and the second electrode, wherein the organic functional layercomprises a light-emitting layer; the light-emitting layer comprising ahost material and a guest material; the host material being an exciplexcomposed of a donor molecule and a receptor molecule; and the donormolecule and/or the receptor molecule containing a plurality of sterichindrance groups.

Optionally, the donor molecule is a compound (having a hole transportproperty) containing at least one of carbazolyl, triphenylaminyl, andaryl. The receptor molecule is a compound (having an electronictransport property) containing at least one of pyrimidinyl, triazinyl,oxadiazolyl, pyridyl, carbazolyl, aryl, cyano, acridinyl,dibenzothiophenyl, triphenylphosphonyl, and triphenylboryl.

Optionally, the donor molecule employs any one of the followingmolecular structures:

wherein X in the above molecular structures is hydrogen or a sterichindrance group, and at least one X is a steric hindrance group.

Optionally, the donor molecule employs any one of the followingstructures:

Optionally, the receptor molecule is a compound containing at least oneof pyrimidinyl, triazinyl, oxadiazolyl, pyridyl, carbazolyl, aryl,cyano, acridinyl, dibenzothiophenyl, triphenylphosphonyl, andtriphenylboryl.

Optionally, the receptor molecule employs any one of the followingmolecular structures:

wherein X in the above molecular structures is hydrogen or a sterichindrance group, and at least one X is a steric hindrance group.

Optionally, the receptor molecule employs any one of the followingstructures:

Optionally, the steric hindrance groups are groups each independentlycontaining substituted or unsubstituted alkyl, cycloalkyl, aryl, silyl,and borosilicate.

Optionally, the steric hindrance group is selected from one or more ofthe structures shown below:

Optionally, the number of steric hindrance groups on the donor moleculestructure or the receptor molecule structure is less than or equal tosix.

Optionally, the mass ratio of the donor molecule to the receptormolecule in the exciplex is 1:9 to 9:1.

Optionally, the mass ratio of the donor molecular material to thereceptor molecular material is 1:2 to 1:5, or the mass ratio of thedonor molecular material to the receptor molecular material is 2:1 to5:1.

Optionally, the guest material is a fluorescent material or aphosphorescent material.

Optionally, the mass ratio of the host material to the guest material is1000:1 to 2:1.

Optionally, the mass ratio of the host material to the guest material is200:1 to 5:1.

The technical solution of the present disclosure has the followingadvantages:

1. The organic electroluminescent device provided by the presentdisclosure comprises a first electrode, a second electrode, and anorganic functional layer located between the first electrode and thesecond electrode. The organic functional layer comprises alight-emitting layer. The light-emitting layer comprising a hostmaterial and an guest material; the host material being an exciplexcomposed of a donor molecule and a receptor molecule; the donor moleculeand/or the receptor molecule containing a steric hindrance group X.

Firstly, by increasing the distance between the donor molecule and thereceptor molecule to reduce the overlapping degree between the HighestOccupied Molecular Orbital (HOMO) and Lowest Unoccupied MolecularOrbital (LUMO) of the formed exciplex body, the singlet-triplet energylevel gap ΔE_(ST) is reduced, thereby increasing the RISC rate(k_(RISC)) of the exciplex body, and facilitating the RISC of excitonsfrom the triplet state to the singlet state. Eventually, more excitonsare converted from the triplet state to the singlet state via RISC,thereby making full use of the triplet energy.

Secondly, if the polarons transfer energy to the triplet state of theexciplex body by means of Dexter energy transfer, the triplet energy ofthe exciplex body may be increased, which in turn causes molecular bondsof the exciplex body to be broken, resulting in a shortened devicelifetime. The introduction of large steric hindrance groups X on thedonor molecule and/or the receptor molecule is able to increase thedistance between molecules of the exciplex body, reduce the tripletconcentration of the host material in the light-emitting layer, andinhibit the Triplet-Triplet Annihilate (TTA) and TPA, thereby enablingto prolong the service life of the electroluminescent device.

Furthermore, the singlet or triplet energy transfer process from thehost material to the guest material contains two energy transfermechanisms, i.e., Förster energy transfer mechanism and Dexter energytransfer mechanism, as shown in FIG. 1 . Compared with the short-rangeDexter energy transfer mechanism, the long-range Förster energy transfermechanism has a longer energy transfer distance. Therefore, theintroduction of the large steric hindrance groups X on the donormolecule and/or the receptor molecule leads to an increased distancebetween molecules of the host material and the guest material so as tohave the exciplex host material transfer energy to the guest materialmainly through the Förster energy transfer mechanism, thereby inhibitingthe Dexter energy transfer, avoiding the high doping concentrationaccompanied with the short-range Dexter energy transfer mode, and alsoavoiding the problems such as efficiency quenching, roll-off, and highcost of the electroluminescent device due to a high dopingconcentration.

2. In the organic electroluminescent device provided by the presentdisclosure, the steric hindrance of the donor molecules and/or thereceptor molecules in the light-emitting layer is increased byintroducing the large steric hindrance groups X of formula X-1-X-22 tothe donor molecules and/or the receptor molecules, so as to increase thedistance between the donor molecules and the receptor molecules, reducethe overlapping degree between HOMO and LUMO, increase the RISC rate(k_(RISC)) of the exciplex host material, enhance the Förster energytransfer, and improve the device efficiency. Moreover, introducing thelarge steric hindrance groups of the above X-1-X-22 is able to reducethe triplet concentration of the host material in the light-emittinglayer, inhibit the TPA, and prolong the device lifetime.

3. In the organic electroluminescent device provided by the presentdisclosure, if a large amount of large steric hindrance groups X areintroduced, the too high molecular weight will lead to a highevaporation temperature of the donor molecules and/or the receptormolecules, increasing the difficulty of evaporation process. Therefore,in the organic light-emitting layer of the OLED device, X in themolecular structure of the donor molecule or the receptor molecule ishydrogen or a steric hindrance group, and at least one X is a sterichindrance group. At least one of the donor molecule and the receptormolecule contains a steric hindrance group, and the number of sterichindrance groups on a single donor molecule or receptor moleculestructure is less than or equal to six, so that the service life of theOLED device is able to be prolonged, and the difficulty of theevaporation process is able to be controlled. When two or more sterichindrance groups are presented in the molecular structure of a donormolecule or a receptor molecule, the structure of each steric hindrancegroup may be different.

BRIEF DESCRIPTION OF THE DRAWINGS

To illustrate the technical solutions in specific exemplary embodimentsof the present disclosure or the prior art more clearly, the drawingsused in the description of the specific exemplary embodiments or theprior art are briefly described below. Apparently, the drawings in thefollowing description are only some exemplary embodiments of the presentdisclosure, and a person of ordinary skilled in the art can obtain otherdrawings according to these drawings without involving any inventiveeffort.

FIG. 1 is a schematic diagram of energy transfer in the luminescenceprocess of an organic electroluminescent device;

FIG. 2 is a schematic structural diagram of an organicelectroluminescent device according to Exemplary embodiment 1 of thepresent disclosure;

FIG. 3 shows molecular structures of materials used in theelectroluminescent device of Exemplary embodiments 1-4 and ComparativeExemplary embodiments 1 and 2 of the present disclosure;

FIG. 4 is an external quantum efficiency-brightness-power efficiencydiagram of Exemplary embodiment 2 and Comparative Exemplary embodiment 1of the present disclosure; and

FIG. 5 is a luminescent spectrum of an organic electroluminescent deviceof Exemplary embodiment 2 and Comparative Exemplary embodiment 1 of thepresent disclosure.

DETAILED DESCRIPTION

In 1987, Deng Qingyun (C. W. Tang) and Vanslyke of the Eastman KodakCompany, U.S.A. first reported a two-layer organic electroluminescentdevice prepared by using a transparent conductive film as an anode, Alq₃as a light-emitting layer, a triarylamine-based material as a holetransport layer, and an Mg/Ag alloy as a cathode. The conventionalfluorescent materials are easy to synthesize and stable and have a longdevice lifetime. However, due to the electron spin inhibition, at most25% of singlet excitons can be used for luminescence, and 75% of tripletexcitons are wasted. The external quantum efficiency of the device isoften less than 5%, which needs to be further improved.

A fluorescent OLED device capable of breaking the limitation of 25% ofthe internal quantum efficiency limitation mainly employs a ThermallyActivated Delayed Fluorescence (TADF) mechanism. The TADF mechanismutilizes an organic small molecular material having a smallsinglet-triplet energy level gap (ΔE_(ST)). The triplet excitons of theorganic small molecular material having a small singlet-triplet energylevel gap can be converted to singlet excitons by the process of ReverseIntersystem Crossing (RISC) under the absorption of ambient heat, whichallows to reach 100% of the internal quantum efficiency of the device intheory. However, the currently reported TADF material has a largeefficiency roll-off and a short service life under high brightness,which limits its application in panchromatic display and white-lightillumination.

The present disclosure provides an organic electroluminescent devicecomprising an organic functional layer, the organic functional layercomprising a light-emitting layer, the light-emitting layer comprising ahost material and a guest material; the host material being an exciplexcomposed of a donor molecule and a receptor molecule, the donor moleculeand/or the receptor molecule containing a plurality of steric hindrancegroups, so as to solve the problems of the large singlet-triplet energylevel gap (ΔEST) of an exciplex TADF host material, the low RISC ratek_(RISC), and the severe Triplet-Polaron Annihilate (TPA) in thelight-emitting layer, thereby improving the device efficiency and theservice life of the organic electroluminescent device.

The technical solutions of the present disclosure are clearly andcompletely described below with reference to the accompanying drawings.It is obvious that the described Exemplary embodiments are a part of theembodiments of the present disclosure, and not all of the embodiments.All other embodiments obtained by a person of ordinary skill in the artbased on the Exemplary embodiments of the present disclosure withoutinvolving an inventive effort are within the protection scope of thepresent disclosure.

In the description of the present disclosure, it is to be noted that theterms “first”, “second”, and “third” are only for descriptive purpose,and cannot be construed as indicating or implying relative importance.

The present disclosure can be implemented in different forms, and shouldnot be construed as being limited to the Exemplary embodiments set forthherein. In contrast, these Exemplary embodiments are provided such thatthe present disclosure will be thorough and complete, and the concept ofthe present disclosure is thoroughly presented to those skilled in theart. The present disclosure is defined only by the appended claims. Inthe accompanying drawings, the dimensions and the relative dimensions oflayers and regions are exaggerated for clarity.

Exemplary Embodiment 1

This Exemplary embodiment provides an organic electroluminescent device,having a first electrode 1, a second electrode 2, and an organicfunctional layer 3 located between the first electrode 1 and the secondelectrode 2, as shown in FIG. 2 . The first electrode 1 is an anode, thesecond electrode 2 is a cathode, and the organic functional layer 3includes a hole injection layer 31, a hole transport layer 32, alight-emitting layer 33, an electron transport layer 34, and an electroninjection layer 35 which are arranged in a stacked manner. That is, thestructure of the organic electroluminescent device is: anode/holeinjection layer/hole transport layer/light-emitting layer/electrontransport layer/electron injection layer/cathode.

The light-emitting layer 33 is composed of a host material and a guestmaterial doped in the host material. The guest material may be afluorescent material or a phosphorescent material. The exciplex is usedas the host material, and the mass ratio of the host material to theguest material is 1000:1 to 2:1, preferably 200:1 to 5:1. The exciplexis composed of a donor molecule and a receptor molecule. The donormolecule is a compound with the hole transport property containing atleast one of carbazolyl, triphenylaminyl, and aryl. The receptormolecule is a compound with the electron transport property containingat least one of pyrimidinyl, triazinyl, oxadiazolyl, pyridyl,carbazolyl, aryl, cyano, acridinyl, dibenzothiophenyl,triphenylphosphonyl, and triphenylboryl. At least one of the donormolecule or the receptor molecule contains a steric hindrance group Xfor increasing the distance between the donor molecule and the receptormolecule, and the steric hindrance groups X each independentlycontaining substituted or unsubstituted alkyl, cycloalkyl, aryl, silyl,and borosilicate.

In this Exemplary embodiment, a compound composed of triphenylaminyl andcarbazolyl is selected as the donor molecule, and the selectedsubstituent group X is

The compound has the structure as shown in formula (1-1):

A compound composed of carbazolyl and triazinyl is selected as thereceptor molecule, and the selected substituent group X is

The compound has the structure as shown in formula (2-34):

In the organic electroluminescent device of this Exemplary embodiment,the first electrode 1 is made of ITO material, The hole injection layer31 is made of 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene(HATCN for short). The hole transport layer 32 is made of a holetransport materialN,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB forshort). The electron transport layer 34 is made of an electron transportmaterial 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi forshort). The electron injection layer 35 is made of an electron injectionmaterial LiF. The second electrode 2 is made of Al. The electroninjection layer 35 and the second electrode 2 form an electron injectionlayer/metal layer structure.

The light-emitting layer 33 is designed such that a donor molecule asshown in formula (1-1) and a receptor molecule as shown in formula(2-34) form an exciplex body. In the exciplex, the mass ratio of thedonor molecule as shown in formula (1-1) to the receptor molecule asshown in formula (2-34) is 2:8. A yellow fluorescent dye PPTPAD isselected as the guest material, and the doped yellow fluorescentmaterial PPTPAD accounts for 1% of the mass of the light-emitting layermaterial to have the organic electroluminescent device form thefollowing specific structure:

ITO/HATCN (5 nm)/NPB (40 nm)/molecule (1-1) (19.8 wt %):molecule (2-34)(79.2 wt %):PPTPAD (1 wt %) (20 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al (150nm).

The layer thickness is presented in parentheses, and the molecularformulas of molecule HATCN, molecule NPB, molecule PPTPAD, and moleculeTPBi are as shown in FIG. 3 .

As an alternative exemplary embodiment, the mass ratio of the donormolecule as shown in formula (1-1) to the receptor molecule as shown informula (2-34) may also be selected to be other values in the range of1:2-1:5, other values in the range of 2:1-5:1, or other values in therange of 1:9-9:1, which are all able to achieve the object of thepresent disclosure and fall within the protection scope of the presentdisclosure.

As an alternative exemplary embodiment, the mass fraction of the dopedyellow fluorescent material PPTPAD in the light-emitting layer materialmay also be selected to be other values in the range of 200:1-5:1 orother values in the range of 1000:1-2:1, which are all able to achievethe object of the present disclosure and fall within the protectionscope of the present disclosure.

As an alternative exemplary embodiment, the donor molecule and thereceptor molecule forming the exciplex are not limited to the molecularstructure as shown in formula (1-1) and the molecular structure as shownin formula (2-34):

An exciplex is able to be formed by any one of the above provided donormolecule structures substituted by any one of the above steric hindrancegroups X and any one of the above provided receptor molecules which arenot substituted by the steric hindrance group X;

An exciplex is able to be formed by any one of the above provided donormolecules which are not substituted by the steric hindrance group X andany one of the above provided receptor molecule structures substitutedby any one of the above steric hindrance groups X; and

An exciplex is able to be formed by any one of the above provided donormolecule structures substituted by any one of the above steric hindrancegroups X and any one of the above provided receptor molecule structuressubstituted by any one of the above steric hindrance groups X.

Exemplary Embodiment 2

In Exemplary embodiment 2, the OLED device may be designed as an organicelectroluminescent device, including an anode, a hole injection layer, ahole transport layer, an organic light-emitting layer, an electrontransport layer, an electron injection layer, and a cathode. In thisExemplary embodiment, a compound composed of triphenylaminyl andcarbazolyl is selected as the donor molecule, and the selectedsubstituent group X is

The compound has the structure as shown in formula (1-1):

A compound composed of carbazolyl and triazinyl is selected as thereceptor molecule and has a structure of formula (A), which differs fromthe molecule (2-34) used in the Exemplary embodiment 1 in that there isno large steric hindrance substituent group, i.e., tert-butyl. Themolecular structure of formula (A) is as follows:

The donor molecule as shown in formula (1-1) and the receptor moleculeas shown in formula (A) form an exciplex. In the exciplex, the massratio of the donor molecule as shown in formula (1-1) to the receptormolecule as shown in formula (A) is 2:8.

Exemplary embodiment 2 differs from Exemplary embodiment 1 only in theaspect of organic light-emitting layer, i.e. the receptor moleculethereof does not contain a large steric hindrance group, and is designedas follows:

ITO/HATCN (5 nm)/NPB (40 nm)/molecule (1-1) (19.8 wt %):molecule (A)(79.2 wt %):PPTPAD (1 wt %) (20 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al (150nm). The fluorescent dye PPTPAD is selected as the doping material,where the doped yellow fluorescent material PPTPAD accounts for 1% ofthe mass of the light-emitting layer material.

Comparative Exemplary Embodiment 1

In Comparative Exemplary embodiment 1, the OLED device may be designedas an organic electroluminescent device, including an anode, a holeinjection layer, a hole transport layer, an organic light-emittinglayer, an electron transport layer, an electron injection layer, and acathode. In this comparative Exemplary embodiment, a compound composedof triphenylaminyl and carbazolyl is selected as the donor molecule, andhas a structure of formula (B), which differs from the donor molecule(1-1) used in Exemplary embodiment 1 in that there is no large sterichindrance substituent group, i.e., tert-butyl. The molecular structureof formula (B) is as follows:

A compound composed of carbazolyl and triazinyl is selected as thereceptor molecule and has a structure of formula (A), which differs fromthe molecule (2-34) used in Exemplary embodiment 1 in that there is nolarge steric hindrance substituent group, i.e., tert-butyl.

The donor molecule as shown in formula (B) and the receptor molecule asshown in formula (A) form an exciplex. In the exciplex, the mass ratioof the donor molecule as shown in formula (B) to the receptor moleculeas shown in formula (A) is 2:8.

Comparative Exemplary embodiment 1 differs from Exemplary embodiments 1and 2 only in the aspect of organic light-emitting layer, i.e. inComparative Exemplary embodiment 1, the donor molecule and the receptormolecule do not contain a large steric hindrance group, and is designedas follows:

ITO/HATCN (5 nm)/NPB (40 nm)/molecule (B) (19.8 wt %):molecule (A) (79.2wt %):PPTPAD (1 wt %) (20 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al (150 nm). Thefluorescent dye PPTPAD is selected as the doping material, where thedoped yellow fluorescent material PPTPAD accounts for 1% of the mass ofthe light-emitting layer material.

Exemplary Embodiment 3

In Exemplary embodiment 3, the OLED device may be designed as an organicelectroluminescent device, including an anode, a hole injection layer, ahole transport layer, an organic light-emitting layer, an electrontransport layer, an electron injection layer, and a cathode. In thisExemplary embodiment, a compound composed of triphenylaminyl andcarbazolyl is selected as the donor molecule, and the selectedsubstituent group X is

The compound has the structure as shown in formula (1-10):

A compound composed of carbazolyl and triazinyl is selected as thereceptor molecule, and the selected substituent group X is

The compound has the structure as shown in formula (2-19):

In Exemplary embodiment 3, the first electrode, i.e., the anode, of theorganic electroluminescent device is made of ITO material. The holeinjection layer is made of2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HATCN forshort). The hole transport layer is made of a hole transport materialN,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB forshort). The electron transport layer is made of an electron transportmaterial 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi forshort). An electron injection material LiF and a cathode material Al areselected to form an electron injection layer/metal layer structure.

The light-emitting layer 33 is designed such that a donor molecule asshown in formula (1-10) and a receptor molecule as shown in formula(2-19) form an exciplex. In the exciplex, the mass ratio of the donormolecule as shown in formula (1-10) to the receptor molecule as shown informula (2-19) is 1:1. A red phosphorescent dye Ir(piq)₃ is selected asthe doping material, and the doped red phosphorescent material Ir(piq)₃accounts for 1% of the mass of the light-emitting layer material to havethe organic electroluminescent device form the following specificstructure:

ITO/HATCN (5 nm)/NPB (40 nm)/molecule (1-10) (49.5 wt %):molecule (2-19)(49.5 wt %):1 wt % Ir(piq)₃ (20 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al (150nm). The molecular structure of the red phosphorescent material Ir(piq)₃is as shown in FIG. 3 .

Exemplary Embodiment 4

In Exemplary embodiment 4, the OLED device may be designed as an organicelectroluminescent device, including an anode, a hole injection layer, ahole transport layer, an organic light-emitting layer, an electrontransport layer, an electron injection layer, and a cathode. In thisExemplary embodiment, a compound composed of triphenylaminyl andcarbazolyl is selected as the donor molecule, and the selectedsubstituent group X is

The compound has the structure as shown in formula (1-10):

A compound composed of carbazolyl and triazinyl is selected as thereceptor molecule, and has a structure of formula (C), which differsfrom the receptor molecule (2-19) used in Exemplary embodiment 3 in thatthere is no large steric hindrance substituent group, i.e., tert-butyl.The molecular structure of formula (C) is as follows:

An exciplex is formed by the donor molecule as shown in formula (1-10)and the receptor molecule as shown in formula (C). In the exciplex, themass ratio of the donor molecule as shown in formula (1-10) to thereceptor molecule as shown in formula (C) is 1:1.

Exemplary embodiment 4 differs from Exemplary embodiment 3 only in theaspect of organic light-emitting layer, i.e. the receptor moleculethereof does not contain a large steric hindrance group, and is designedas follows:

ITO/HATCN (5 nm)/NPB (40 nm)/molecule (1-10) (49.5 wt %):molecule (C)(49.5 wt %):1 wt % Ir(piq)₃ (20 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al (150nm). A red phosphorescent dye Ir(piq)₃ is selected as the dopingmaterial, where the doped red phosphorescent material Ir(piq)₃ accountsfor 1% of the mass of the light-emitting layer material.

Comparative Exemplary Embodiment 2

In Comparative Exemplary embodiment 2, the OLED device may be designedas an organic electroluminescent device, including an anode, a holeinjection layer, a hole transport layer, an organic light-emittinglayer, an electron transport layer, an electron injection layer, and acathode. In this Exemplary embodiment, a compound composed oftriphenylaminyl and carbazolyl is selected as the donor molecule, andhas a structure as shown in formula (B):

A compound composed of carbazolyl and triazinyl is selected as thereceptor molecule, and has a structure of formula (C), which differsfrom the receptor molecule (2-19) in Exemplary embodiment 3 in thatthere is no large steric hindrance substituent group, i.e., tert-butyl.

The donor molecule as shown in formula (B) and the receptor molecule asshown in formula (C) form an exciplex. In the exciplex, the mass ratioof the donor molecule as shown in formula (B) to the receptor moleculeas shown in formula (C) is 1:1.

Comparative Exemplary embodiment 2 differs from Exemplary embodiments 3and 4 only in the aspect of organic light-emitting layer, i.e. inComparative Exemplary embodiment 2, the donor molecule and the receptormolecule do not contain a large steric hindrance group, and is designedas follows:

ITO/HATCN (5 nm)/NPB (40 nm)/molecule (B) (49.5 wt %):molecule (C) (49.5wt %):1 wt % Ir(piq)₃ (20 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al (150 nm). Ared phosphorescent dye Ir(piq)₃ is selected as the doping material,where the doped red phosphorescent material Ir(piq)₃ accounts for 1% ofthe mass of the light-emitting layer material.

Exemplary Embodiment 5

This Exemplary embodiment differs from Exemplary embodiment 1 in thatthe donor molecule has a structure as shown in formula (1-7), the massratio of the donor molecule material to the receptor molecule materialis 1:9, and the mass ratio of the host material to the guest material is200:1. The device has an external quantum efficiency of 12.6% and a CIEcolor coordinate (0.32, 0.59) at a brightness of 5000 cd/m².

Exemplary Embodiment 6

This Exemplary embodiment differs from Exemplary embodiment 1 in thatthe donor molecule has a structure as shown in formula (1-10), the massratio of the donor molecule material to the receptor molecule materialis 1:2, and the mass ratio of the host material to the guest material is200:1. The resulting device has an external quantum efficiency of 14.1%and a CIE color coordinate (0.32, 0.59) at a brightness of 5000 cd/m².

Exemplary Embodiment 7

This Exemplary embodiment differs from Exemplary embodiment 1 in thatthe donor molecule has a structure as shown in formula (1-13), the massratio of the donor molecule material to the receptor molecule materialis 1:1, and the mass ratio of the host material to the guest material is5:1. The resulting device has an external quantum efficiency of 12.4%and a CIE color coordinate (0.32, 0.59) at a brightness of 5000 cd/m².

Exemplary Embodiment 8

This Exemplary embodiment differs from Exemplary embodiment 1 in thatthe donor molecule has a structure as shown in formula (1-16), the massratio of the donor molecule to the receptor molecule is 2:3, and themass ratio of the host material to the guest material is 2:1. Theresulting device has an external quantum efficiency of 13.6% and a CIEcolor coordinate (0.32, 0.59) at a brightness of 5000 cd/m².

Exemplary Embodiment 9

This Exemplary embodiment differs from Exemplary embodiment 2 in thatthe donor molecule has a structure as shown in formula (1-18), the massratio of the donor molecule to the receptor molecule is 1:3, and themass ratio of the host material to the guest material is 100:1. Theresulting device has an external quantum efficiency of 14.6% and a CIEcolor coordinate (0.32, 0.59) at a brightness of 5000 cd/m².

Exemplary Embodiment 10

This Exemplary embodiment differs from Exemplary embodiment 2 in thatthe donor molecule has a structure as shown in formula (1-25), the massratio of the donor molecule to the receptor molecule is 1:4, and themass ratio of the host material to the guest material is 150:1. Thedevice has an external quantum efficiency of 15.1% and a CIE colorcoordinate (0.33, 0.59) at a brightness of 5000 cd/m².

Exemplary Embodiment 11

This Exemplary embodiment differs from Exemplary embodiment 2 in thatthe donor molecule has a structure as shown in formula (1-26), the massratio of the donor molecule to the receptor molecule is 1:4, and themass ratio of the host material to the guest material is 150:1. Theresulting device has an external quantum efficiency of 13.8% and a CIEcolor coordinate (0.33, 0.59) at a brightness of 5000 cd/m².

Exemplary Embodiment 12

This Exemplary embodiment differs from Exemplary embodiment 3 in thatthe receptor molecule has a structure as shown in formula (2-5), themass ratio of the donor molecule to the receptor molecule is 2:1, andthe mass ratio of the host material to the guest material is 500:1. Theresulting device has an external quantum efficiency of 14.2% and a CIEcolor coordinate (0.67, 0.33) at a brightness of 5000 cd/m².

Exemplary Embodiment 13

This Exemplary embodiment differs from Exemplary embodiment 3 in thatthe receptor molecule has a structure as shown in formula (2-16), themass ratio of the donor molecule to the receptor molecule is 1:1, andthe mass ratio of the host material to the guest material is 150:1. Thedevice has an external quantum efficiency of 17.1% and a CIE colorcoordinate (0.67, 0.33) at a brightness of 5000 cd/m².

Exemplary Embodiment 14

This Exemplary embodiment differs from Exemplary embodiment 4 in thatthe receptor molecule has a structure as shown in formula (2-17), themass ratio of the donor molecule to the receptor molecule is 1:1, andthe mass ratio of the host material to the guest material is 150:1. Thedevice has an external quantum efficiency of 17.7% and a CIE colorcoordinate (0.67, 0.33) at a brightness of 5000 cd/m².

Exemplary Embodiment 15

This Exemplary embodiment differs from Exemplary embodiment 1 in thatthe receptor molecule has a structure as shown in formula (2-20), themass ratio of the donor molecule to the receptor molecule is 6:4, andthe mass ratio of the host material to the guest material is 90:1. Thedevice has an external quantum efficiency of 13.9% and a CIE colorcoordinate (0.33, 0.59) at a brightness of 5000 cd/m².

Exemplary Embodiment 16

This Exemplary embodiment differs from Exemplary embodiment 1 in thatthe receptor molecule has a structure as shown in formula (2-25), themass ratio of the donor molecule to the receptor molecule is 3:7, andthe mass ratio of the host material to the guest material is 60:1. Thedevice has an external quantum efficiency of 14.8% and a CIE colorcoordinate (0.33, 0.59) at a brightness of 5000 cd/m².

Exemplary Embodiment 17

This Exemplary embodiment differs from Exemplary embodiment 2 in thatthe receptor molecule has a structure as shown in formula (2-26), themass ratio of the donor molecule to the receptor molecule is 4:6, andthe mass ratio of the host material to the guest material is 40:1. Thedevice has an external quantum efficiency of 15.8% and a CIE colorcoordinate (0.33, 0.59) at a brightness of 5000 cd/m².

Exemplary Embodiment 18

This Exemplary embodiment differs from Exemplary embodiment 3 in thatthe receptor molecule has a structure as shown in formula (2-31), themass ratio of the donor molecule to the receptor molecule is 1:8, andthe mass ratio of the host material to the guest material is 100:1. Thedevice has an external quantum efficiency of 18.1% and a CIE colorcoordinate (0.67, 0.33) at a brightness of 5000 cd/m².

Exemplary Embodiment 19

This Exemplary embodiment differs from Exemplary embodiment 3 in thatthe receptor molecule has a structure as shown in formula (2-32), themass ratio of the donor molecule to the receptor molecule is 1:7, andthe mass ratio of the host material to the guest material is 180:1. Thedevice has an external quantum efficiency of 17.9% and a CIE colorcoordinate (0.67, 0.33) at a brightness of 5000 cd/m².

Exemplary Embodiment 20

This Exemplary embodiment differs from Exemplary embodiment 4 in thatthe receptor molecule has a structure as shown in formula (2-35), themass ratio of the donor molecule to the receptor molecule is 1:6, andthe mass ratio of the host material to the guest material is 80:1. Thedevice has an external quantum efficiency of 16.9% and a CIE colorcoordinate (0.67, 0.33) at a brightness of 5000 cd/m².

Exemplary Embodiment 21

This Exemplary embodiment differs from Exemplary embodiment 4 in thatthe receptor molecule has a structure as shown in formula (2-36), themass ratio of the donor molecule to the receptor molecule is 1:1, andthe mass ratio of the host material to the guest material is 60:1. Thedevice has an external quantum efficiency of 18.0% and a CIE colorcoordinate (0.67, 0.33) at a brightness of 5000 cd/m².

In the foregoing Exemplary embodiments, the selectable structuralformulas and the substitutable positions of the steric hindrance group Xof the donor molecule are as shown in any one of formulas (D-1) to(D-9). The selectable structural formulas and the substitutablepositions of the steric hindrance group X of the receptor molecule areas shown in any one of formulas (A-1) to (A-12). In the foregoingExemplary embodiments, X in the molecular structure of the donormolecule or the receptor molecule is hydrogen or a steric hindrancegroup, and at least one X is a steric hindrance group. At least one ofthe donor molecule and the receptor molecule contains a steric hindrancegroup, and the number of steric hindrance groups on a single donormolecule or receptor molecule structure is less than or equal to six.When two or more steric hindrance groups are present on the molecularstructure of the donor molecule or the receptor molecule, the structureof each steric hindrance group may be different. In the foregoingExemplary embodiments, the steric hindrance group X may be selected fromone or more of the structures shown in formulas (X-1) to (X-22).

In the foregoing Exemplary embodiments, the structure of the donormolecule may be any one of the structures shown in formulas (1-1) to(1-27).

In the foregoing Exemplary embodiments, the structure of the receptormolecule may be any one of the structures shown in formulas (2-1) to(2-36).

In the foregoing Exemplary embodiments, the mass ratio of the donormolecule to the receptor molecule in the exciplex is 1:9 to 9:1.Preferably, the mass ratio of the donor molecular material to thereceptor molecular material is 1:2 to 1:5, or, the mass ratio of thedonor molecular material to the receptor molecular material is 2:1 to5:1.

Test Exemplary Embodiment 1

The characteristics of current, voltage, brightness, and luminescencespectrum of the devices of Exemplary embodiments 1-4 and ComparativeExemplary embodiments 1 and 2 are synchronously tested by a PR 650spectral scanning brightness meter and a Keithley K 2400 digital sourcemeter system.

I. The performances of the OLED devices of Exemplary embodiment 1,Exemplary embodiment 2, and Comparative Exemplary embodiment 1 aretested. The test results are shown in Table 1 below:

TABLE 1 Performance test of the OLED devices External Power DeviceBrightness quantum efficiency lifetime CIE color (cd/m²) efficiency(lm/W) LT90 coordinate Exemplary 5000 16.6% 26.6 1.4 (0.32, 0.59)embodiment 1 Exemplary 5000 16.1% 25.8 1.3 (0.32, 0.59) embodiment 2Comparative 5000 11.8% 16.1 1 (0.33, 0.59) Exemplary embodiment 1

As can be seen from Table 1, in the OLED devices prepared through thematerials provided in Exemplary embodiment 1, Exemplary embodiment 2,and Comparative Exemplary embodiment 1, the guest materials are 1% dopedfluorescent material PPTPAD. The external quantum efficiency and thepower efficiency of the OLED devices of Exemplary embodiment 1 andExemplary embodiment 2 are higher than those of the OLED device ofComparative Exemplary embodiment 1 at the same brightness. The externalquantum efficiency and the power efficiency of the OLED device ofExemplary embodiment 1 are higher than those of the OLED device ofExemplary embodiment 2, since each of the donor molecule and thereceptor molecule provided in Exemplary embodiment 1 contains a largesteric hindrance group. This further indicates that the donor and/orreceptor material containing the large steric hindrance group enables toincrease the inter-molecular distance between the donor molecule and thereceptor molecule, decrease the overlapping degree between the HOMO andLUMO orbitals forming the exciplex body, and reduce the singlet-tripletenergy level gap ΔE_(ST), thereby increasing the RISC rate (k_(RISC)) ofthe exciplex body, enhancing the Föster energy transfer to the guestmolecule, and improving the efficiency of the organic electroluminescentdevice.

As shown in FIG. 4 , the ordinate is the external quantum efficiency andthe power efficiency, and the abscissa is the brightness. Theelectroluminescence performance of the OLED device of Exemplaryembodiment 2 is better than that of the OLED device of ComparativeExemplary embodiment 1 at the same brightness. This indicates that thedevice performance under the exciplex host material with a large sterichindrance group X (tert-butyl) is better than device performance underthe exciplex host material without a large steric hindrance group, whichis reflected in a higher external quantum efficiency and powerefficiency. FIG. 5 shows the luminescence spectra of the organicelectroluminescent devices of Exemplary embodiment 2 and ComparativeExemplary embodiment 1.

The lifetimes LT90 of Exemplary embodiment 1, Exemplary embodiment 2 andComparative Exemplary embodiment 1 are tested at a constant brightnessof 5000 cd/m², and Comparative Exemplary embodiment 1 is defined as astandard device having a lifetime of 1 equivalent. As can be seen fromTable 1, the OLED devices of Exemplary embodiment 1 and Exemplaryembodiment 2 both have an improved lifetime compared to the OLED deviceof Comparative Exemplary embodiment 1, and the OLED device of Exemplaryembodiment 1 has the longest lifetime. Therefore, the introduction of alarge steric hindrance group enables to effectively reduce the tripletconcentration in the light-emitting layer, inhibit the TPA, and prolongthe device lifetime.

II. The performances of the OLED devices of Exemplary embodiment 3,Exemplary embodiment 4, and Comparative Exemplary embodiment 2 aretested. The test results are shown in Table 2 below:

TABLE 2 Performance test of the OLED devices External Power DeviceBrightness quantum efficiency lifetime CIE color (cd/m²) efficiency(lm/W) LT90 coordinate Exemplary 5000 18.4% 14.8 1.3 (0.67, 0.33)embodiment 3 Exemplary 5000 17.6% 13.2 1.1 (0.67, 0.33) embodiment 4Comparative 5000 15.2% 11.8 1 (0.67, 0.33) Exemplary embodiment 2

As can be seen from Table 2, in the OLED devices prepared through thematerials provided in Exemplary embodiment 3, Exemplary embodiment 4,and Comparative Exemplary embodiment 2, the guest luminescent materialsare 1% doped phosphorescent material Ir(piq)₃. The external quantumefficiency and the power efficiency of the OLED devices of Exemplaryembodiment 3 and Exemplary embodiment 4 are higher than those of theOLED device of Comparative Exemplary embodiment 2 at the samebrightness. This indicates that the donor and/or receptor moleculecontaining the large steric hindrance group enables to decrease theoverlapping degree between the HOMO and LUMO orbitals forming theexciplex body, and reduce the singlet-triplet energy level gap ΔE_(ST),thereby increasing the RISC rate (k_(RISC)) of the exciplex body,enhancing the Föster energy transfer to the guest molecule, improvingthe efficiency of the organic electroluminescent device, and achieving abetter performance of the phosphorescent luminescent device. Since boththe donor molecule and the receptor molecule have the large sterichindrance group X, the OLED device of Exemplary embodiment 3 has thebest performance. Moreover, the doping concentration of the guestmaterial is only 1%, which is low and thus results in a reduced devicecost.

The lifetimes LT90 of Exemplary embodiment 3, Exemplary embodiment 4 andComparative Exemplary embodiment 1 are tested at a constant brightnessof 5000 cd/m², and Comparative Exemplary embodiment 2 is defined as astandard device having a lifetime of 1 equivalent. As can be seen fromTable 2, the OLED devices of Exemplary embodiment 3 and Exemplaryembodiment 4 both have an improved lifetime (LT90 is tested at aconstant brightness of 5000 cd/m²) compared to the OLED device ofComparative Exemplary embodiment 2, which indicates that theintroduction of the large steric hindrance group enables to effectivelyreduce the triplet concentration in the light-emitting layer, inhibitthe TPA, and prolong the device lifetime.

It is apparent that the foregoing Exemplary embodiments are merelyillustrated for clarity, and not intended to limit the embodiments.Other different forms of variations or modifications may also be made bya person skilled in the art on the basis of the foregoing description.There is no need and no way to exhaust all the embodiments herein.Obvious variations or modifications resulting therefrom still fallwithin the protection scope of the present disclosure.

What is claimed is:
 1. An organic electroluminescent device, the organicelectroluminescent device comprising an organic functional layer, theorganic functional layer comprising a light-emitting layer; thelight-emitting layer comprising a host material and a guest material;the host material being an exciplex composed of a donor molecule and areceptor molecule; the donor molecule and/or the receptor moleculecontaining a plurality of steric hindrance groups, wherein the sterichindrance groups are groups each independently containing substituted orunsubstituted cycloalkyl, silyl, boryl and borosilicate, wherein thedonor molecule employs any one of the following structures:


2. The organic electroluminescent device according to claim 1, whereinthe receptor molecule is a compound containing at least one ofpyrimidinyl, triazinyl, oxadiazolyl, pyridyl, carbazolyl, aryl, cyano,acridinyl, dibenzothiophenyl, triphenylphosphonyl, and triphenylboryl.3. The organic electroluminescent device according to claim 2, whereinthe receptor molecule employs any one of the following molecularstructures:

wherein X in the molecular structures is hydrogen or a steric hindrancegroup, and at least one X is a steric hindrance group.
 4. The organicelectroluminescent device according to claim 3, wherein the receptormolecule employs any one of the following structures:


5. The organic electroluminescent device according to claim 1, wherein anumber of steric hindrance groups on the donor molecule structure or thereceptor molecule structure is less than or equal to six.
 6. The organicelectroluminescent device according to claim 1, wherein a mass ratio ofthe donor molecule to the receptor molecule in the exciplex is 1:9 to9:1.
 7. The organic electroluminescent device according to claim 6,wherein the mass ratio of the donor molecular material to the receptormolecular material is 1:2 to 1:5, or the mass ratio of the donormolecular material to the receptor molecular material is 2:1 to 5:1. 8.The organic electroluminescent device according to claim 1, wherein amass ratio of the host material to the guest material is 1000:1 to 2:1.9. The organic electroluminescent device according to claim 8, whereinthe mass ratio of the host material to the guest material is 200:1 to5:1.
 10. The organic electroluminescent device according to claim 1,wherein the steric hindrance groups comprise:


11. The organic electroluminescent device according to claim 1, whereinthe steric hindrance groups are only selected from:


12. The organic electroluminescent device according to claim 1, whereinthe steric hindrance groups are only selected from:


13. The organic electroluminescent device according to claim 1, whereinthe steric hindrance groups comprise:


14. An organic electroluminescent device, the organic electroluminescentdevice comprising an organic functional layer, the organic functionallayer comprising a light-emitting layer; the light-emitting layercomprising a host material and a guest material; the host material beingan exciplex composed of a donor molecule and a receptor molecule; thedonor molecule and/or the receptor molecule containing a plurality ofsteric hindrance groups, wherein the steric hindrance groups are groupseach independently containing substituted or unsubstituted cycloalkyl,silyl, boryl, and borosilicate, wherein the donor molecule employs anyone of the following structures:


15. An organic electroluminescent device, the organic electroluminescentdevice comprising an organic functional layer, the organic functionallayer comprising a light-emitting layer; the light-emitting layercomprising a host material and a guest material; the host material beingan exciplex composed of a donor molecule and a receptor molecule; thedonor molecule and/or the receptor molecule containing a plurality ofsteric hindrance groups, wherein the steric hindrance groups are groupseach independently containing substituted or unsubstituted cycloalkyl,silyl, boryl, and borosilicate, and wherein the receptor moleculeemploys any one of the following molecular structures:

wherein X in the molecular structures is hydrogen or a steric hindrancegroup, and at least one X is a steric hindrance group, wherein the donormolecule employs any one of the following structures:


16. The organic electroluminescent device according to claim 15, whereinthe receptor molecule employs any one of the following structures: